1. Field of the Invention
The present invention relates to Global Positioning Systems (GPS) and, more particularly, to those utilizing dual Global Positioning receivers.
2. Description of the Prior Art
GPS is well known in the prior art. The systems utilize a plurality of satellites which broadcast, among other data, their exact location along with Universal Time Coordinated (UTC) to indicate when the broadcast position was valid. A GPS receiver located on an aircraft, for example, receives the broadcast from several satellites and determines aircraft body rates, accelerations, pseudo range as well as aircraft position to a high degree of accuracy and transmits this data, along with a time indication for showing when the data was valid, to, for example, the Flight Management System (FMS) aboard the aircraft. Many aircraft also utilize information from independent sources (e.g. gyroscopes and accelerometers) to provide position, velocity and attitude information to the FMS. While the attitude information from gyroscopes and accelerometers is quite accurate, position and velocity information may grow somewhat less accurate over time because of drift. The GPS solution error, however, is more constant regardless of flight time. More sophisticated systems, may utilize both GPS rate information and an independent source of rate information (such as gyroscopes and accelerometers) to produce a "Hybrid" signal. For example, the Honeywell HG2001AC01, comprise an Inertial Reference Unit (IRU) which includes inertial reference hardware such as gyroscopes and accelerometers that provide position, attitude and rate information (including a cyclical interrupt to which all internal processes are synchronized and from which an indication as to when the information is valid can be derived) and an IRU processing block which may be an 80960 processor, with attendant memory, timing hardware and operational flight software. The processing block, receives the information from the gyroscopes and accelerometers, including the cyclical interrupts, and also receives the GPS information, including time marks, from the GPS receivers, and operates to produce the "Hybrid" signal which is utilized by the FMS. The time mark signal produced by the GPS receiver in conjunction with the UTC (exact time) of the time mark which is broadcast to the IRS by the receiver over a communication bus (such as the ARINC 429) is used to synchronize the incoming GPS data with the internal IRS data including the cyclical interrupt. The hybrid signal is thus generated using time correlated IRS and GPS data.
Because the GPS information can become invalid or unusable (as indicated, for example, by the absence of a time mark signal), it may be desirable to use redundant GPS receivers. While three or more GPS receivers could be used, the HG2001AC01 is set up to utilize two GPS receivers so that if one of them becomes invalid or unusable, the other one may be utilized. This redundant GPS receiver system requires that when the primary GPS receiver becomes unusable and the secondary GPS receiver is being employed, the condition of the primary GPS receiver be rechecked periodically to determine if it can later be used again. Accordingly, the HG2001AC01 uses a switching scheme in the form of a multiplexor to receive the information and the time mark from both GPS receivers (one at a time) and to pass the signals from the selected GPS receiver on to the system processor where the GPS information is combined with the IRS information to produce the hybrid information for the FMS. Upon the incapacitance of the primary GPS receiver, the multiplexor switches the secondary GPS receiver to the system processor for use during the time that the primary GPS receiver is not being used. During this time, a periodic reversionary check of the primary GPS receiver is performed to see if it is again usable and if so it switches the primary GPS receiver back to being the source of information for producing the Hybrid signals.
Unfortunately, every time that a channel switch is made, a validation procedure must be run before the data can be assimilated for use in the hybrid solution and up to ten seconds of secondary channel data can thus be lost.